High frequency tantalum attenuation in traveling wave tubes



Oct. 21,1969 I R.P.ANAND 3,474,284

HIGH FREQUENCY TANTALUM ATTENUA'IION IN TRAVELING WAVE TUBES Filed-Dec. 27, 1966 2 Sheets-Sheet 1 I. Q 'i FIG. I

l INVENTOR RP ANAND R. P. ANAND Oct. 21, 1969 2 .Shets-Sheet 2 Filed Dec. 27, 1966 United I States Patent Int. Cl. H01j 25/34 US. Cl. 315 3.5 8 Claims ABSTRACT OF THE DISCLOSURE The high frequency attenuator in a traveling wave tube and in other devices, in which the attenuator issubject disadvantageously to electron and/or ion bombardment, comprises a coating containing the element tantalum, e.g., either pure tantalum or tantalum nitride.

This is a continuation-in-part of application, Ser. No. 498,930, filed Oct. 20, 1965, now abandoned, and relates to radio-frequency attenuation devices, and more particularly to the attenuation of reflected radio-frequency en ergy in traveling wave tubes.

.In the traveling wave tube, an electron beam is, projected in close proximity to a slow-wave structure, such as a conductive helix, with interaction taking place between the electron beam and the field of an electromagnetic wave propagatingon the helix. With the helix many wavelengths long at the operating frequency, .the eumulative interaction between the electron beam and the field of the wave results in a transfer of energy from the beam to the field of the wave, thereby amplifying the wave.

recognized, and the tubes widely used, it is also recognized that the device can become unstable; that is oscillate,

"ice

A more specific object of the present invention is to reduce the effects of sustained particle bombardment of the attenuation coating of a traveling wave tube.

In an illustrative embodiment of this invention, there is provided a traveling wave tube comprising an electron gun for projecting a beam of electrons toward a collector. An elongated slow-wave structure, such as a wire helix, which extends between the electron gun and the collector, surrounds the beam path and is coupled at its input end to a signal source and at its output end to a load. The tube is provided with a focusing structure for constraining the flow of electrons to a path entirely within the slow- Wave structure.

Feedback energy is attenuated by a coating of lossy material on a portion of the helix. As mentioned before, there has been for some time a need for a helix loss material capable of extended operation without change of loss characteristic, but not until recently was the cause of the aforementioned deterioration determined to be electron and ion bombardment of the loss material. It is well known that the helix loss material should have a low vaporization pressure, a high melting point, and should preferably be a material having comparatively high resistivity.

- In accordance with the invention, a portion of the slowwave structure is coated with a film of tantalum or tantalum nitride for attenuating feedback energy. Both tantalum and tantalum nitride possess a sufiiciently high melting point, low vaporization pressure, and high resistivity to meet the above requirements. Most importantly, however, I have found that a thin film of either tantalum or tantalum nitride on the helix and support rods provide the desired loss characteristics without deteriorating as" a result of electron and ion bombardment.

These and. other objects and features of the invention will be better appreciated from a consideration of the While the advantages of traveling wave tubes are well I if energy propagation-along the helix is not carefully controlled. The major cause of instability in a traveling wave tube is the reflection of electromagnetic wave energy due to impedance mismatches in the output, section of the helix. Subsequent and repeated reflections, of course, cause oscillations in the tube. These reflected waves are usually suppressed by an attenuation coating along. Part of the helix. It is common practice to include on part of the helix a lossy material, typically a thin coating of graphite or carbon composition, which absorbs and dissipates the reflected Wave energy. The input signal energy is maintained through the loss section because at that location much of the input signal wave energy exists on the beam as spacecharge waves and can therefore propa-, gate freely. It has been observed-however, that the loss characteristics in tubes of this type tend to deteriorate in time, such deterioration being the result of susained-electron and ion bombardment of the attenuation coating. The deterioration of the loss characteristics causes the traveling wave tube to self-oscillate and thus renders it completely useless.

In a long life and high gain traveling wave tube it is particularly important that the helix loss characteristic be maintained throughout the operating life of the tube. This requirement is necessary because the loss material is very carefully applied during the manufacture to minimize interference with propagating space-charge. waves while effectively absorbing reflected electromagnetic waves on the helix. I i

It is an object of this inventionto provide traveling wave tubes of longer life..

I lum, tantalum nitride andcarbon films.

following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a partial sectional view of a traveling wave tube employing the principles of the invention;

FIG. 2 is a section taken along line 2-2 of FIG. 1;

FIG. 3 is an enlarged view of part of the helix and helix support structure of the traveling wave tube of FIG. 1;

and a a FIG. 4 is a graph comparing the effect of electron bombardment on the attenuation characteristics of tanta-- Referring now to FIG. 1, there is shown a traveling Wave tube package 10, the purpose of which is to amplify electromagnetic Waves which are transmitted tothe tube p by an input waveguide 11 as shown by the arrow. Located near the opposite end of the device is an output wave guide 12 for abstracting amplified electromagnetic waves from the traveling wave tube and transmitting them to an appropriate load as indicated by the other arrow. Extending between the input and output waveguides is a con ductive wire helix 13 which surrounds theflcentral axis of the device. An electron beam is formed and projected along the central axis by an electron gun 14 of a Wellknown type which includes a cathode 15, abeam-forming.

electrode 16 and an accelerating anode 17. Theelectron beam is collected by a collector electrode 18 located at the end of the helix 13 opposite the electron gun 14.

. The electron beam is projected in close proximity to the conductive helix with interaction taking place between the electron beam and the field of an electromagnetic.

' the field of the wave, thereby amplifying the Wave.

The helix 13 is mounted by three support rods 19 which are shown in more detail in FIGS. 2 and 3. A periodic magnetic structure 20 surrounding both the rods 19 and the helix 13 focuses the beam and constrains it to flow along the central axis to the collector 18. Other known focusing structures can be used, if so desired.

As mentioned previously, the major cause of instability in a traveling wave tube is the tendency of wave energy to be reflected from the output end of the helix 13 back toward the input end and thereby to cause oscillations within the device. The reflected waves are usually suppressed by an attenuation coating along part of the helix 13. It has been common practice to include on part of the helix a lossy material, typically a thin coating of graphite or carbon composition, which absorbs and dissipates the reflected wave energy. Such helix loss materials should have a low vaporization pressure, a high melting point, and should preferably be a material having comparatively high resistivity. The resistivity of graphite at 20 C., for instance, is 800 microhm-cm.

In accordance with one embodiment of the invention, the helix 13 and the support rods 19 are coated along a discrete region with a layer 21 of tantalum or tantalum nitride for absorbing reflected electromagnetic wave energy; this is best seen in FIG. 3. Both tantalum and tantalum nitride have a sufiiciently high melting point, low vaporization pressure, and resistivities of about microhm-cm. and 100 microhmcm., respectively, at C. to meet the above preferred characteristics. The lower resistivity of tantalum means that tantalum films are made correspondingly thinner in order to attain a desired attenuation. A typical thickness of the tantalum layer is 500 angstrom units.

The manner in which the tantalum or tantalum nitride layer 21 attenuates the reflected energy is best understood by considering a single turn of the helix 13 in the loss region defined by the layer 21. As reflected electromagnetic wave energy travels from the output end of the helix 13 toward the input end, it encounters the turn of the helix and therein produces an electric field. The ends of the turn are electrically connected through the layer 21 on the support rods 19. The electric fields produced by the reflected waves in the turn of the helix are effectively shunted through the layer 21 and thereby attenuated. Thus the layer 21 acts as a load to the electric fields in the helix in much the same way that a resistor acts as a load when connected across a battery. Also of importance is the attenuation of surface currents produced in the helix by the reflected energy. Since these currents travel on the helix surface, they are attenuated by the layer 21 on the surface of the helix itself.

' It is important that the loss material be very carefully applied 50 that it effectively acts as a reflectionless absorber of waves traveling toward the input end, while minimally affecting the growing wave traveling toward the output end. To this end, the density of the loss material should be tapered in the direction of the output end so that reflected-backward traveling waves are gradually absorbed. On the other hand, the total length of the loss section should be minimized to give the smallest possible interference with the growing wave. To obtain the proper loss pattern, tantalum is deposited preferably by means of a sputtering chamber comprising an electrode having a cylindrical inner surface coated with tantalum which surrounds the helix. The helix is masked so as to define a loss region and the masks are flared to allow a tapered transition of resistance of the tantalum film. Tantalum is sputtered onto the helix by establishing a gaseous discharge between the electrode and the helix. By similar techniques well known in the art, tantalum nitride films can be sputtered onto the helix.

' In a long life and high gain traveling wave tube it is particularly important that the helix loss characteristic be maintained throughout the operating life of the tube; This requirement is necessary because the loss material is very carefully applied during the manufacturing to minimize interference with propagating space-charge waves while effectively absorbing electromagnetic waves on the helix. It has been found that in conventional tubes utilizing graphite or other carbon compositions as a loss material,the loss characteristic tends to change during extended tube operation. It has further been found that this change results from electron and ion bombardment of the graphite or carbon composition which in time reduces the wave loss and tends to make the tube unstable and completely useless. I have found, however, that a sputtered tantalum or tantalum nitride film on helix and support rods provides the desired loss characteristics without deteriorating as a result of particle bombardment.

As yet the exact mechanism whereby electron bombardment accelerates the deterioration of carbon film characteristics is not completely understood. It is likely that present vacuum firing techniques 'do not release all adsorbed hydrogen and hydrocarbons found in graphite and that subsequent sustained electron bombardment drives off these gases. The hydrogen thereby released is ionized by the electrons of the beam. The ions thereby formed collide with, and dislodge or sputter away, carbon atoms in the graphite film. The sputtering rate of carbon in a hydrogen discharge; that is, hydrogen ionized by an electric field, is 262 milligrams per amp-hour, a relatively rapid 'rate; however, it is not known if this rate is valid for the particular carbon composition used on the helix and at the possible partial pressures of hydrogen existent in the tube. In contrast, the sputtering rate of tantalum in a hydrogen discharge is only 16 mg./amp-hour, reflecting the fact that tantalum is much heavier than carbon and therefore is not so easily dislodged by colliding hydrogen ions.

A second possibility is a cyclic reaction in which carbon monoxide and/or carbon dioxide, which are released from the thermionic cathode, dissociate under electron bombardment to release atomic oxygen which attacks the graphite film. The atomic oxygen combines with carbon atoms of the graphite forming carbon monoxide and/or carbon dioxide which in turn dissociate under electron bombardment, thus beginning the cycle again. The erosion of evaporated carbon film by oxygen atoms produced in a discharge tube is described by J. Strenznewski and J. Turkevich, in the paper The Reaction of Carbon with Oxygen Atoms, Proc. of the 3rd Carbon Conference, pp. 273-287. They find that the reaction rate depends directly on oxygen atom concentration and is independent of the temperature in the range 20-100 C. What role electron bombardment plays in accelerating this reaction is not clear; however, the dissociation of CO by electron-impact has been observed by W. W.-

Lozier, Physics Review 46, page 268 (1934), and H. D. Hagstrum, On the Dissociation Energy of Carbon Monoxide and the Heat of Sublimation of Carbons, Physics Review 72, pp. 947-963 (1947). The following equations give the observed ionization and dissociation processes and the corresponding onset potentials, the minimum electron energy necessary to cause dissociation of carbon monoxide:

has the additional advantage that it is substantially unaffected by oxygen at all.

The effect of electron bombardment on the attenuation characteristics of several helix loss materials is shown in FIG. 4. Tests were performed on traveling Wave tubes having helices coated with the following loss materials: (I) sputtered tantalum film; (II) sputtered tantalum film having length and film thickness both less than those of the tantalum film of (I); (III) sputtered tantalum nitride film; (IV) and (V) sprayed Aquadag; and (VI) pyrolytically deposited carbon film. The tubes were operated at normal voltages, with the output terminated and no R.F. excitation. The electron beam was perturbed by distorting the magnetic focusing field in the attenuator section. The electrons then undergo elastic scattering with gas molecules in the tube resulting in electron bombardment of the loss material. FIG. 4 (V) shows graphically the role of electron bombardment in causing deterioration of graphite films. During the interval between 76 and 122 hours, the electron beam was focused properly, and the attenuation during this interval did not change. Outside of that interval, however, the electron beam was perturbed, resulting in a decrease in attenuation of the graphite film. It is clear from the test results shown in FIG. 4 that only the sputtered tantalum (I and II) and tantalum nitride (III) films have loss characteristics that are substantially unaffected by prolonged electron and ion bombardment.

Tubes with helices; having sputtered tantalum and tantalum nitride films have been successfully life tested. For example, traveling wave tubes operated under normal conditions, with five watts of power being dissipated in a tantalum nitride attenuator, have exhibited no substantial degradation for 2000 hours. Thus, traveling wave tubes in accordance with the invention remain stable for periods of time far in excess of those previously attainable.

Both tantalum and tantalum nitride are also useful as loss materials on other slow-wave structures and, in fact, in any device, such as a magnetron or cross-field amplifier, in which the loss material is subjected to sustained particle bombardment. Numerous other embodiments and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention.

In particular, at present the nose cones of space vehicles are coated with pyrolytic carbon which upon reentry reacts With the ionized atmosphere. The use of either tantalum or tantalum nitride, on the other hand, would not so react, thereby maintaining the integrity of the nose cone.

What is claimed is:

1. In combination:

a transmission system for propagating energy in a first direction and including a source of electrons and means for projecting said electrons for interaction with said propagating energy;

means forming a discontinuity in said transmission system capable of disadvantageously reflecting energy in a second direction opposite to the first direction;

means for absorbing the reflected energy thereby preventing the establishment of oscillations in said transmission system, said absorbing means being subject to electron and/ or ion bombardment;

said absorbing means comprising a coating including the element tantalum, said coating being free of other metallic deposits and having loss characteristics that are substantially unaffected by prolonged electron and/or ion bombardment thereof. 2. The combination of claim 1 wherein said coating comprises substantially pure tantalum.

3. The combination of claim 1 wherein the coating comprises substantially tantalum nitride.

4. The combination of claim 1 wherein: said transmission system comprises a slow-wave structure of a traveling wave tube. 5. An electron discharge device comprising: a source of electrons; an electron collector; means for projecting the electrons in the form of a beam from said source to said collector; means for focusing the beam of electrons; a slow-wave structure surrounding the beam path and support means therefor; said slow-wave structure having input and output ends; means for coupling electromagnetic wave energy to the input end of said slow-wave structure; means for extracting electromagnetic wave energy from the output end of said slow-wave structure; said output end having a discontinuity which inherently reflects wave energy; means for attenuating the reflected wave energy along a discrete portion of said slow-wave structure; said attenuating means being subject to electron and/ or ion bombardment; said attenuating means comprising a thin film having the element tantalum and having loss characteristics that are substantially unaffected by prolonged electron and/or ion bombardment thereof, said thin film being free of other metallic deposits, said slowwave structure being in intimate contact with said thin film along substantially the entire length of said discrete portion. 6. The electron discharge device of claim 5 wherein: said slow-wave structure comprises a wire helix; said support means comprises a plurality of rods each substantially coextensive with said :helix and abutting against said helix; said thin film is deposited onto a portion of said helix and said support rods. 7. The electron discharge device of claim 5 wherein said thin film comprises substantially pure tantalum.

8. The electron discharge device of claim 5 wherein said thin film comprises substantially tantalum nitride.

References Cited UNITED STATES PATENTS 3,368,103 2/1968 Thall 3l5-3.5 2,497,496 2/ 1950 Gooskens et al. 252-512 X 2,720,609 10/1955 Bruck et al. 3153.5 2,771,565 11/1956 Bryant et a1. 315-35 HERMAN K. SAALBACH, Primary Examiner -S. CHATMON, 1a., Assistant Examiner U.S. Cl. X.R. 

